1. Field of the Invention
This invention relates to flow-velocity measuring apparatus of the type wherein a cylindrical object in a stream of flowing fluid produces a wake including a distinct pattern of vortices known as the Karman vortex street. The vortices are shed alternately from opposite sides of the object in a periodic manner. There is a definite relationship between the frequency f of shedding of the vortices, the velocity V of the stream, and the height h of the object in the direction transverse to the direction of the flowing fluid, expressed by: EQU f = KV/h
where K is a constant for flow within a range of velocities. Accordingly, within this range, it is possible to easily determine the flow velocity V by measuring the frequency f of the generation of vortices.
Description of the Prior Art
It has been found that the desired linear relationship between flow-velocity V and frequency f of the vortex generation does not apply over the full range of conditions encountered in flow measurement. When a cylindrical object is immersed in a stream of fluid flowing through a pipe and particularly when flow velocity increases beyond a certain point, k is not a constant and the measurement of vortex production does not simply yield a measurement of velocity. Though the causes of this disparity may not have yet been completely explained, it is believed to result from the uneven velocity distribution of a fluid flowing through a pipe, from the fluctuation and eddies other than Karman vortices which develop in the flowing fluid, and from changes in the separating point of the boundary layer of the fluid flowing along the object surface as the flow velocity increases.
Various arrangements have been proposed for generating Karman vortices more stably in a flow measurement environment. In one such type of arrangement disclosed in U.S. Pat. No. 3,564,915 to Miyaji Tomota, et al., and in U.S. patent application Ser. No. 348,121 filed Apr. 5, 1973, by Hiroo Yamasaki, et al., a cylindrical element is formed with a transverse bore or slit intercepting the cylinder surface in the regions where boundary layer separation normally occurs, with fluid flow alternating through the transverse bore or slit as the Karman vortices are generated and assisting in causing or retarding bondary layer separation for more reliable vortex production without influence from eddies and other fluid fluctuations.
In another prior art arrangement disclosed in U.S. Pat. No. 3,693,438 to Hiroo Yamasaki et al., a cylindrical element is formed with recessed surface portions to control boundary layer separation for more reliable vortex production.
In another prior art arrangement disclosed in U.S. Pat. no. 3,116,639 to Bird, a cylindrical object is formed with a cross-sectional shape which reduces flow resistance at high speeds, and avoids deleterious effects on vortex production caused by cavitation, such shapes including a streamlined fore-part (such as a semi-ellipse) with the after part cut away and recessed.
Apparatus of the general type referred to above has not been fully satisfactory in all conditions and fluid velocity measurement applications in providing a linear relationship between flow velocity V and the frequency f of vortex generation. While gains have been made in improving stability of vortex production, difficulty in attaining linearity at very high velocities of flow persists.